Canon REBEL SL1 EOS 100D Instruction Manual page 41

Troubleshooting Guide
In shooting modes (<___._ > <l_> <_tl)> <_> <'_> <_i>) whose
default setting is <$A> (Auto flash), the built-in flash will rise
automatically when necessary.
If you shoot continuously with the built-in flash at short intervals, the
flash may stop operating to protect the flash unit.
If you use a flash unit other than an EX-series Speedlite, the flash will
always be fired at full output (p.312).
Under [1_2: Flash control] and [External flash C.Fn setting], if
[Flash metering mode] is set to [TTL], the flash will always fire at full
output (p.233).
If flash exposure compensation has already been set with the external
Speedlite, flash exposure compensation cannot be set with the
camera. When the external Speedlite's flash exposure compensation
is canceled (set to 0), flash exposure compensation can be set with
the camera.
Under [1_2: Flash control], set [Flash sync. speed in Av mode] to
[Auto] (p.232).
The built-in flash's pop-up mechanism moves slightly. This is normal
and not a malfunction.
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